1. Field of the Invention
The present invention relates generally to methods and systems for measuring a dimensional change of a material in response to a change in environment. Particularly, the methods and apparatus of the present invention relate to measuring a dimensional change of coal in response to exposure to a gas, such as, for instance, carbon dioxide at a selected pressure.
2. State of the Art
Subterranean coal seams may contain substantial quantities of natural gas, primarily in the form of methane. The methane may be contained within the coal as adsorbed gas and various techniques have been developed to enhance removal of the methane therefrom. However, the rate of recovery of methane from coal seams may typically depend on the rate at which gas can flow through the coal seam to a production well. The flow rate of methane through a coal seam may be affected by many factors including, for instance, the porosity of the coal, the permeability of the coal, the extent, if any, of the fracture system which exists within the coal seam, and the stress therein.
Typically, the naturally occurring system of fractures within a coal seam does not provide for an acceptable methane recovery rate. Therefore, in general, coal seams must be stimulated to enhance the recovery of methane from the seams. Techniques for increasing the methane production rate often attempt to increase the permeability of the coal, which will permit an increase in the rate of production of methane from the coal seam.
For instance, one technique for stimulating the coal formation in the near wellbore region involves the fracturing of the coal bed by injecting under substantial pressure an aqueous mixture with suitable entrained particles as propping agents to open up fracture planes and channels in which the particles settle out to prop the fractures open as they are formed. Such entrained particles are commonly termed “proppants,” which may comprise sand grains, man-made or specially engineered proppants, resin-coated sand, or ceramic materials. Although hydraulic fracturing of coal seams may be effective in increasing the permeability of the coal, fracturing fluids may cause, in the long term, a loss in methane productivity due to adsorption of the fracturing fluids onto the coal fracture surfaces. Adsorption of the fluid may cause swelling of the coal itself, which may plug the coal cleat or natural fracture system and inhibit recovery of methane therefrom.
Another technique to stimulate coal bed methane production from a coal seam is to inject a gas, such as air, nitrogen, ammonia, or carbon dioxide, into the coal seam. This process may be commonly referred to as “enhanced coal bed methane (ECBM) production.” This technique has been utilized in the past to degasify coal mines for safety reasons. For instance, U.S. Pat. No. 3,384,416 discloses such a technique where a refrigerant fluid with proppants is injected into the coal seam to fracture the coal. The injected refrigerant fluid and methane may escape from a borehole drilled into the coal under its own pressure or the injected refrigerant fluid and methane may be removed via pumps.
U.S. Pat. No. 4,083,395 discloses a technique for recovering methane from a coal seam where a carbon dioxide-containing fluid is introduced into the coal deposit through an injection well and held therein for a period sufficient to enable a substantial amount of methane to be desorbed from the surfaces of the coal deposit. Following the so-called hold period, the injected carbon dioxide-containing fluid and desorbed methane may be recovered through a recovery well or wells spaced from the injection well. The process is repeated until sufficient methane has been removed to enable safe mining of the coal deposit.
Of course, there must be at least one injection well and at least one production well for the enhanced in situ degasification of coal deposits. More preferably, a suitable plurality of injection wells and a plurality of gas production wells may be formed within the coal deposit. The position of the plurality of injection wells and the plurality of gas production wells may be selected for maximum economy in recovery of the methane contained therein.
In addition, besides being reservoirs for methane, coal beds have enormous carbon dioxide storage potential. Therefore, in addition to any methane that may be produced from a coal bed, coal beds have been considered as a storage mechanism for retaining carbon dioxide gas. Such carbon dioxide sequestration may be employed for storing carbon dioxide produced as a by-product of industry or as otherwise may be desirable.
However, pertaining to either methane production or carbon dioxide sequestration, it has been found that injection of gasses may cause the coal to swell, which may reduce the permeability of the coal. Of course, coal swelling may also affect the stress state of the coal and may further affect the permeability of the coal. Further, and as mentioned above, the permeability of the coal may influence methane production from a coal bed or carbon dioxide sequestration within a coal bed.
The swelling or expansion of certain coals as a function of elevated temperature is a well-known and studied characteristic. This swelling behavior, also referred to as dilation, may be related, although not precisely, to the volatility of the coal. However, the suitability of any particular coal for gas production may be more accurately determined from knowledge of the actual swelling characteristics of the coal, rather than from the volatile matter content of the coal, since the swelling property may be a characteristic more precisely related to changes in permeability.
A conventional apparatus for measuring the temperature dependent swelling behavior (i.e., coefficient of thermal expansion) of materials is a dilatometer. One example of a conventional dilatometer is disclosed in U.S. Pat. No. 4,923,307 to Gilmore et al. Additionally, modified conventional dilatometers have been used for measuring dimensional changes in other materials, and sometimes under controlled conditions. For instance, an article, published in Rev. Sci. Instrum., 1979, and titled DILATOMETER FOR THE IN SITU OBSERVATION OF HIGH-TEMPERATURE HIGH-PRESSURE HYDROGEN ATTACK by A. A. Sagüés describes a test apparatus comprising an autoclave within which a sample and a control sample may be disposed. The autoclave chamber may be heated, and hydrogen may be introduced thereinto. Also, disposed within the autoclave chamber is a capacitive displacement sensor, which indicates the relative expansion of the sample. Electrical signals from the sensor are carried through the wall of the autoclave chamber via an electrical feedthrough.
Other examples of modified dilatometers may be found in U.S. Pat. No. 6,476,922 and U.S. patent application Ser. No. 10/293,342, both to Pananelli. Both U.S. Pat. No. 6,476,922 and U.S. patent application Ser. No. 10/293,342 relate to a dilatometer including at least two optical systems which are able to focalize, with a predetermined degree of magnification, the images of two ends of the test piece. The apparatuses are structured to perform measurement of a size of a test piece while completely eliminating any influence on such measurement by the holder or the measuring system.
Unfortunately, however, while a number of authors have proposed models that attempt to relate coal swelling characteristics to permeability changes, the inventors of the present invention are aware of very little data published on the swelling properties of coal under different gas environments which is available for incorporation into mathematical models. More particularly, if the relationship between swelling or shrinkage of coal in relationship to different gases and pressures were more easily determined experimentally, mathematical modeling of the injection process, for either methane production or carbon dioxide sequestration, may be improved. Furthermore, the injection process itself may correspondingly benefit by way of more accurate predictive models thereof.
One conventional approach for measuring the swelling characteristics of coal under different gas environments is to utilize one or more strain gages affixed to the surface of a coal sample. Such a conventional procedure requires affixing a strain gage adhesively to the surface of a coal sample. It has been noted that the conventional adsorption or desorption process may be extremely slow, taking nearly three months for the coal matrix strains to stabilize. As a further note, a strain gauge may have an elasticity that is not negligible with respect to the elasticity of the coal sample to which it is affixed, and thus may introduce artifact (i.e., extraneousness or inaccuracy) into the strain gauge reading.
In view of the foregoing problems and shortcomings with existing apparatus, methods, and systems for testing and modeling the dimensional behavior of permeable materials such as coal, as well as the need for data useful for modeling such materials, it may be desirable to provide improved methods and apparatus for determining the dimensional behavior of permeable materials, particularly as related to pressurized gas environments.